Gains that fluctuate rapidly across time and frequency result in audible artifacts in digital audio processing systems.
U.S. Pat. No. 6,351,731 describes an adaptive filter featuring a speech spectrum estimator receiving as input an estimated spectral magnitude signal for a time frame of the input signal and generating an estimated speech spectral magnitude signal representing estimated spectral magnitude values for speech in a time frame. A spectral gain modifier receives as input an initial spectral gain signal and generates a modified gain signal by limiting a rate of change of the initial spectral gain signal with respect to the spectral gain over a number of previous time frames. The modified gain signal is then applied to the spectral signal, which is then converted to its time domain equivalent.
U.S. Pat. No. 6,088,668 describes a noise suppressor, which includes a signal to noise ratio (SNR) determiner, a channel gain determiner, a gain smoother and a multiplier. The SNR determiner determines the SNR per channel of the input signal. The channel gain determiner determines a channel gain per the ith channel. The gain smoother produces a smoothed gain per the ith channel and the multiplier multiplies each channel of the input signal by its associated smoothed gain.
U.S. Pat. No. 7,016,507 describes a noise reduction algorithm with the dual purpose of enhancing speech relative to noise and also providing a relatively clean signal for the compression circuitry. In an embodiment, a forgetting factor is introduced to slow abrupt gain changes in the attenuation function.